A radio frequency (RF) device such as an RF filter is typically configured using a connection structure of multiple resonators. Such a resonator is a circuit element that resonates at a specific frequency based on a combination of an inductor L and a capacitor C as an equivalent electronic circuit, and each resonator is structured such that a dielectric resonance (DR) element or metallic resonance element is installed inside a cavity such as a metallic cylinder or rectangle, etc., surrounded by a conductor. Thus, each resonator allows existence of only an electromagnetic field of a unique frequency in a processing frequency band in the cavity, enabling microwave resonance. Generally, the resonator has a multi-stage structure including sequentially connected multiple resonance stages, each of which is formed for multiple cavities.
FIG. 1 illustrates an example of a conventional 6-pole bandpass filter 10. Referring to FIG. 1, in the conventional example, the bandpass filter 10 includes a housing 110 having, for example, six cavities sectioned by a predetermined interval or space inside hexahedral metal, and in each cavity, six dielectric or metallic resonance element 122 having high quality factor (Q) values are fixed using a support. Input and output connectors 111 and 113 mounted on a side of the housing 110 and a cover 160 for shielding an open surface of the housing 110 are also provided in the bandpass filter 10. Each cavity of the housing 110 is sectioned by a partition 130 having predetermined-size windows 131 through 135 formed therein to adjust the amount of coupling between resonators, and an inner surface of the housing 110 is silver-plated to stabilize electric performance and to maximize conductivity. A coupling screw 175 that is insertable into the windows 131-135 through the cover 160 or the housing 110 is also provided for fine adjustment of the amount of coupling.
Each resonance element 122 is supported by the support provided erect on a bottom surface, and a tuning screw 170 for tuning a frequency is installed above each resonance element 122 in such a way to be inserted into the cavity through the cover 160 and thus, fine adjustment of a resonant frequency may be possible by frequency tuning with the tuning screw 170.
On a side of the housing 110 are provided the input and output connectors 111 and 113 which are connected to input and output feeding lines (not shown), respectively, in which the input feeding line delivers a signal input from the input connector to a resonance element on the first stage and the output feeding line delivers a signal input from a resonance element on the last stage to the output connector.
An example of an RF filter having the above-described structure is disclosed in a Korean Patent Laid-Open Gazette No. 10-2004-100084 (entitled “Radio Frequency Filter”, published on Dec. 20, 2004, and invented by Jongkyu Park, Sangsik Park, and Seuntaek Chung) filed by the present applicant.
However, in the conventional bandpass filter (or band rejection filter), to construct a filter having multiple poles, a coupling means for coupling multiple cavities with each resonance element 122 is inevitably needed. That is, in the conventional filter, one resonance element 122 implements only a single resonance mode, and thus to implement a multi-mode filter, a structure in which multiple resonators are connected is required. As a result, a significantly large space is needed for implementation of the multi-mode filter, increasing the size, weight, and manufacturing cost of the filter.
As such, a filter having a multi-mode resonator structure is one of communication facilities that occupy large spaces, and research has been steadily and actively performed to reduce the size and weight of the filter. Moreover, in line with a recent trend where each base station has evolved into a small (or micro) cell to respond to high processing speed and improved quality in the recent mobile communication market, the small size and light weight of the filter are required more crucially.